This invention relates to an improved method for the production of waterproof corrugated paperboard.
In the production of corrugated paperboard, the adhesive conventionally employed to bond the corrugated liner to the facing sheets is a Stein-Hall starch-based adhesive in which about five-sixths of the starch is in the form of an uncooked aqueous starch suspension which is brought to the desired viscosity and provided with the requisite tack by a cooked starch paste, which also acts as a carrier to maintain the uncooked starch in suspension. The cooked portion is prepared separately and then blended with the uncooked starch slurry, which conventionally also contains caustic soda, to reduce the gelation time and temperature of the uncooked starch, and borax, to increase the tack of the adhesive. See U.S. Pat. Nos. 3,151,996 and 2,833,662. The starch is ordinarily cooked in a conventional manner, i.e., batch-wise by heating an aqueous slurry so as to gradually bring the temperature of the slurry to gelation temperature. Several processes have also been developed for cooking the starch slurry substantially instantaneously. See U.S. Pat. Nos. 2,717,213; 3,133,836; 3,228,781; 3,308,037; and 3,450,549. U.S. Pat. No. 3,228,781 employs such an instantaneous starch cooking method in the production of a Stein-Hall corrugated board adhesive. See also U.S. Pat. No. 2,609,326.
Because of the economic advantages, considerable effort has been directed to the production of a commercially acceptable flour-based adhesive. See U.S. Pat. Nos. 2,051,025; 2,102,937; 2,212,557; 2,291,586; 2,466,172; 2,529,597; 2,881,086; 2,999,028; 3,163,549; 3,251,703; 3,775,144; and 3,775,145.
When a high performance adhesive is desired, a high amylose (about 80%) starch instead of pearl starch is employed in the cooked or carrier portion of the adhesive. In our prior application Ser. No. 119,904, filed Feb. 8, 1980, whose disclosure is incorporated herein by reference, we disclose a method for converting pearl starch into a modified starch which produces a carrier having the properties of a carrier produced from a high amylose starch.
Several problems are associated with the production of waterproof corrugated paperboard. One of these problems is the variations in waterproofness of the board which occur during a commercial run or from run to run. This is attributed in part to the poor storage life of the adhesive after the resin is added thereto. To overcome this defect, a few mini-batch systems have been installed based primarily on the philosophy that due to poor storage life the resin is added continuously, thereby providing freshly prepared adhesive at the machine. The few mini-batch systems being utilized at the present time are essentially the same even though different engineering techniques are being used to accomplish the same purpose. They are equipped with a variable speed proportioning pump in order to add resin to a specified adhesive flow. The combined resin and adhesive are pushed through a static blender and empty into a 40-100 gallon tank which feeds the corrugating machine. The feed tank is maintained between predetermined high and low liquid levels with a load cell, probes or pressure sensitive high and low cells, such as a DP cell. The flow can be controlled with automatic valves or by actuating and stopping the pumping system. Whatever engineering technique is used, the principle of the proportioning pump adding the proper amount of resin to a controlled flow of basic adhesive and then mixing through a static blender in preparing a small operating quantity of waterproof adhesive is basic to all of them.
Such mini-batch systems, while solving the problems associated with the short storage life of waterproofing adhesives, do not solve other problems associated with the production of waterproof board.
There is a considerable tonnage of waterproof corrugated board manufactured which is used for shipping produce, wet iced poultry, meat boxes, and a variety of other end uses where the board is exposed to water or high humidity. Conventionally, waterproof corrugated adhesive is prepared in the Henry Pratt system. Although there are a variety of techniques used, they all are related to a higher rate of carrier application and higher solids content. In order to waterproof the corrugated paperboard, a conventional waterproofing resin is added to the Stein-Hall adhesive. A wide variety of such resins, e.g., recorcinol formaldehyde resins, urea formaldehyde resins and ketone aldehyde resins, e.g., acetone formaldehyde resins, can be employed. An improved cross-linking resin is the subject of U.S. Pat. No. 3,994,424, whose disclosure is incorporated herein by reference.
The degree of waterproofing is directly related to the rate of adhesive application. We have found that the increase in waterproofing is a linear function relative to rate of adhesive application up to 8 lbs of dry adhesive pickup per 1,000 sq. ft. of board and levels off thereafter so that little improvement in waterproofness occurs above 8 lbs/1,000 sq. ft. of board. The resin is added when the adhesive is finished, mixed for 15 min. and then pumped to storage. Here it is used and, when switching from waterproof to domestic board, the adhesive in storage continues to increase in viscosity. The general practice is to add water to reduce the viscosity, which adversely effects both the machine operation and board quality.
In order to achieve an acceptably high rate of resin application, higher amounts of adhesive than is used for conventional corrugated board is applied when making waterproof board because concentrations of resin high enough to permit the use of the same amount of adhesive magnify the storage instability problems.
Several other problems are associated with conventional methods for producing waterproof board. One of these problems is that the resin has not completely cured when it comes off the corrugator. Therefore, it has been standard practice within the industry to allow the finished board off the machine to remain stacked for 12 hours in order to achieve resin cure. In the industry, this is referred to as "curing in the hot stack." This necessarily adversely affects the economics of production.
Another problem is the strength of the waterproof board. An examination of conventionally produced waterproof board shows that because of the high amount of water added at the double backer station going directly into the double backer over, where pressure is exerted against the board in order to achieve heat transfer, all of the flutes are crushed at the double backer facing. This reduces top-to-bottom compression and reduces stacking strength, which therefore demands much heavier weight board in order to meet specifications.
Another problem associated with the application of larger amounts of water to the paper, either through the use of larger amounts of adhesive or the addition of water to the finished adhesive to maintain proper viscosity or other properties, is that the production rate of waterproof board usually is substantially slower than conventional board.
The aforesaid problems are avoided by the production of corrugated paperboard according to the process of this invention.